STATEMENTS OF INCOME (UNAUDITED) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
STATEMENTS OF INCOME (UNAUDITED) [Abstract]        
Operating revenues $ 471,889 $ 522,921 $ 1,192,578 $ 1,337,419
Operating expenses        
Electric fuel and purchased power 259,783 325,930 676,079 850,583
Operating and maintenance expenses 60,676 61,651 185,961 187,136
Demand side management program expenses 3,129 4,658 9,366 12,469
Depreciation and amortization 28,690 27,067 84,785 79,589
Taxes (other than income taxes) 12,102 10,730 35,142 31,825
Total operating expenses 364,380 430,036 991,333 1,161,602
Operating income 107,509 92,885 201,245 175,817
Other (expense) income, net (73) 286 (53) 432
Allowance for funds used during construction - equity 1,670 894 5,062 4,178
Interest charges and financing costs        
Interest charges - includes other financing costs of $721, $763, $2,243 and $2,188, respectively 17,649 16,456 51,290 48,322
Allowance for funds used during construction - debt (1,052) (655) (3,208) (3,022)
Total interest charges and financing costs 16,597 15,801 48,082 45,300
Income before income taxes 92,509 78,264 158,172 135,127
Income taxes 34,262 29,683 58,271 51,636
Net income $ 58,247 $ 48,581 $ 99,901 $ 83,491